1. Field of the Invention
The present invention relates to the field of thyristors and thyristor-like devices.
2. Prior Art
Thyristors or thyristor-like devices are a general family of semiconductor devices that exhibit bistable characteristics and can be switched between a high impedance, low current off state and a low impedance, high current on state. Such devices are frequently used for current switching in high power applications, as the low impedance on state and the high impedance off state minimize the power dissipation in either operating state. Obviously, however, further reductions in the on impedance are desirable to reduce the power dissipation by such devices and to potentially increase the current carrying capacity thereof. Also, further increases in the off impedance will further reduce the reverse bias leakage current, a counterproductive current in most applications.
A good discussion of the principles and structures of thyristors is provided in Chapter 4, starting on page 190, of "Physics of Semiconductor Devices", second edition, by S. M. Sze. Such devices are generally four layer pnpn silicon devices having three pn junctions in series. An anode connection is provided through an ohmic contact with the first (p) layer, and a cathode connection is provided by an ohmic contact to the fourth (n) layer. Gate connections may be provided to either or both the second (n) layer and the third (p) layer, though normally in a three terminal triggerable device, the third or gate connection is made to the third (p) layer. The basic structure provides a thyristor like device in the form of a triggerable unidirectional switch, though alternate configurations of the basic structure can provide bi-directional switches and related devices (see chapter 4 of Sze referred to above for some examples of such other thyristor-like devices).